THEOIMES OF TEMPEIJATURE CHANGES. 259 



110 substantial basis of supijort either in geological facts or astronomical prin- 

 ciples. In the same chapter more will be said than has been done in the 

 preceding part of the volume, on the climatological eft'ect of the vaiying 

 distribution of land and water on the earth's surface at difierent geological 

 epochs, this last-mentioned cause of climatic change having been prominently 

 put forward by some writers in connection with tlie phenomena of the Gla- 

 cial epoch. Tlie extreme improbability of nny such changes having taken 

 place on a sufficiently large scale to produce effects of more than local im- 

 portance has already been fully set tbrth in a preceding section ; how far 

 such variations as might have occurred would have been likely to aid in 

 bringing about the descril)ed phenomena of glaciation will be considered in 

 tlie course of the next chapter. 



We may now proceed to a Ijrief review of the various theories which seem 

 properljf to come up for discussion in the present C(ninection, beginning with 

 those of the first group mentioned above. 



The first theory which inny be examined is that of Poisson, to the effect 

 that the earth, together with the solar system to which it belongs, wanders 

 til rough regions of space having different temperatures. This is an idea 

 which may properly be designated as purely fancii'ul. The ad\antage which 

 it offers is, that it may be made to fit any possible comljination of facts, so 

 long as local variations of climate are not required to be accounted for. If 

 we wish to explain how it is that, as has been set forth in the preceding 

 pages, the earth's surface has been constantly losing in temperature, we have 

 only to imagine that the solar system has been gradually and steadily mov- 

 ing away from a warmer region in space toward a cooler one. 



This idea, however captivating it might be supposed to be on account of 

 its simplicity, has hardl}' found any favor at the hands of either astronomers 

 or geologists. The former maintain, almost uuanimousl}-, that it is incon- 

 sistent with the principles of their science, and that there is every reason to 

 believe that, as long as the present solar system holds together, the earth will 

 be dependent on the sun for its climate. 



The next theory to be considered is one which has been advocated under 

 various forms and with various modifications. It is to the effect that the 

 position of the earth's axis has been shifted from time to time during the geo- 

 logical ages, either by the shock of a comet, or by the alteration of the posi- 

 tion of the centre of gravity caused by the elevation of mountain chains, or 

 in some other way not specially defined. With this tUeory may be included 



